Completely cured concrete stones are not renowned for their aesthetic qualities particularly when laid as paving. Accordingly, concrete stones are more often than not subjected to secondary processing, of various kinds. One of the forms of secondary processing used in the case of concrete paving stones to enable them more readily to simulate cobble stones made of natural stone is ageing. In effect, the ageing process involves treating the stones in such a way that stone pieces of various shapes and sizes are randomly removed from their edges. The edges of the stones therefore exhibit a "broken" appearance that simulates the edges of cobble stones made of natural stone, that can be referred to as the "cobble" effect. The action of removing material from stones to simulate natural stones and to enhance their aesthetic appearance is known as "distressing".
Whilst ageing can be achieved in a number of ways, for example by shot blasting, hammering and rumbling, rumbling has been the method of choice for many years.
Rumbling involves the batch feeding of a multiplicity of stones that have been fully cured on a pallet into a large rotating drum. In this specification, "fully cured" means that the so-called green stones coming on their pallets (eg 40 to a pallet) from the moulding apparatus have been subjected initially to partial-curing in a curing chamber at a predetermined humidity and temperature and for a predetermined time and then finally to complete curing in open air of the factory yard again for a predetermined time period. The rotation of the "rumbling" drum causes the stones to impact upon one another resulting in their respective top and bottom peripheral edges being randomly distressed to achieve the cobble effect. Because of the magnitude of the impact forces that occur on the stones in the rotating drum, it is essential that the stones are fully cured because partially-cured stones would disintegrate.
Unfortunately rumbling is too random and the impact forces generated by the rotation of the drum can be unpredictable thereby causing some stones to be broken or rendered too small for commercial use of the side faces defined by the vertical edges to be broken which renders laying difficult and/or the peripheral top and bottom edges are broken so that the aesthetic effect is impaired. So, with rumbling, there is sometimes an unacceptably high level of wastage. Moreover, rumbling is expensive because of the high manufacturing, operating and maintenance costs of the rotating drum.
Other disadvantages of rumbling are that instead of a continuous process from the moulding apparatus to the yard from whence the stones can be transported to site, the stones have to be transported in the yard twice which of course increases costs. Moreover, the stones do not emerge from the drum in an orderly fashion and therefore have to be re-stacked on their pallets which interrupts the whole process again and further increases costs.
EP Patent Specification Nos. 0 339 308 and the applicants' own Belgian Patent No. 1010944A3 (EP 0 860 258 A1) overcome the problems of interrupting the continuous process which would otherwise occur from moulding apparatus to the yard by having to re-stack the fully cured aged stones on the pallets after emerging from the drum by ageing partially-cured stones.
In EP Specification No. 0 339 308, machined partially-cured concrete blocks on pallets are aged by hammering tools that have blunt or rounded-off striking surfaces which knock off the top peripheral edges of the blocks leaving the top sides of the concrete blocks smooth.
The applicants' Belgian specification is specifically directed to overcoming the disadvantages of the disruption caused by having fully to cure the concrete stones before being able to coherence rumbling and the concrete stones being presented in a disorderly fashion after emerging from the rotating rumbling drum, necessitating re-stacking on pallets. To this end, the applicants devised a method of ageing concrete stones which comprises beating an array of partially-cured stones in side-by-side and edge-to-edge abutment between two elements (preferably in the form of two plates of which at least one plate is subjected to a vibratory movement). in order to prevent the stones in the array from tilting laterally and/or sliding or rolling over each other, the beating elements are spaced apart by a distance that does not exceed twice the thickness of the stones and the stones are held in abutment in the array by the use of lateral supports or guides and end supports. Whilst the method disclosed in the applicants' Belgian specification overcomes the aforementioned disadvantages and eliminates the breakages that occur with rumbling and the costs incurred by the use of a rotating rumbling drum, the applicants have found in practice that the quality the ageing is not satisfactory, This is because there is minimal distressing of the top and bottom peripheral edges of the stones so that, in the case of paving for example, the finished stones do not simulate natural cobble stones.
Accordingly, the applicants looked at ways in which the quality of the ageing of concrete stones, ie the distressing of the top and bottom peripheral edges, could be improved whilst maintaining the advantages of the method disclosed in their Belgian patent specification and of rumbling, but without the disadvantages of applicants' earlier method and of rumbling.